The base material from which gypsum board is manufactured is the hemihydrate form of calcium sulfate (gypsum), commonly termed stucco, which is produced by the heat conversion of the dahydrate from which the water phase has been removed. It is reported that more than 80% of the more than 23 million metric tons produced each year are converted to the hemihydrate state for use in formulating various building materials such as wall (gypsum) board.
Gypsum board absorbs water, which reduces the strength of the wallboard. Previous attempts to instill water-resistant properties to gypsum board include incorporation of asphalt wax additives. The resulting materials were difficult to use and the core properties difficult to control. This led the industry to move away from asphalt/wax based systems and towards polysiloxane based systems as the moisture preventative of choice. Although these materials provided superior moisture resistance and overcame certain problems associated with asphalt/wax systems, they were both expensive and difficult to use. Economics drove the industry to find a more cost-effective method of controlling both moisture and core performance. The result was the incorporation of a wax emulsion comprised of two different types of wax emulsified with a polyvinylalcohol system.
However, the polyvinylalcohol system suffers from a number of deficiencies, including instability and subjection to rapid separation when stationary which requires mixing prior to use. This also affects the core properties due to the stratification of the compounds in the formulation; the solids content is lower than desired which is a function of the chemistry of this system. In addition, because of destabilization into different phases, there is also the potential for bacterial growth which raises system maintenance and personnel issues at a consuming site.
Thus, a need exists for a stable, water-resistant gypsum composition.